The present invention relates to a fiber-reinforced metallic member in which an essential part of the member is reinforced by inorganic fibers such as Al.sub.2 O.sub.3, SiC, Carbon, etc., and more particularly to a fiber-reinforced metallic member in which improvement of a strength in a boundary portion between a base metal layer (non-reinforced layer) and a reinforced layer is contemplated.
Fiber-reinforced metal (FRM) implies a metal in which reinforcement of a base metal is contemplated by making reinforcing fibers intervene in the base metal, and as a proportion of intervening of fibers is increased, the strength is increased and a proportion of elongation thereof approaches to a proportion of elongation of the fibers. Therefore, it is possible to make the characteristics of the fiber-reinforced metal coincide with aimed values by varying a proportion of intervening of the fibers (a fiber volume proportion (V.sub.f)).
However, in the event that a metallic member is locally fiber-reinforced with a high volume proportion of fibers, at the boundary portion between a base metal layer and a reinforced layer, the characteristics of the metallic member would change abruptly, and hence practically unfavorable phenomena are liable to occur. For instance, paying attention to coefficients of thermal expansion, in contrast to the fact that a coefficient of thermal expansion of Al-alloys is about 22.times.10.sup.-6 /.degree.C., that of Al.sub.2 O.sub.3 is 7.7.times.10.sup.-6 /.degree. C., that Al.sub.2 O.sub.3 is 7.7.times.10.sup.-6 /.degree.C., that of of SiC is 3.1.times.10.sup.-6 /.degree.C., and a coefficient of thermal expansion of inorganic fibers is very small compared with Al-alloys, so that if the metallic member is subjected to large thermal change, then a large difference in thermal expansion would occur between the base metal layer and the reinforced layer depending upon a content proportion in volume of fibers, and there is a possibility that cracks may arise in the boundary portion between the respective layers.
For the purpose of avoiding such abrupt change in characteristics, it would be effective to set a fiber volume proportion (V.sub.f) in the entire reinforced layer low, or to set a fiber volume proportion (V.sub.f) in the reinforced layer low in the portion adjacent to the base metal layer. However, in the case where a fiber shaped body (preform) for complexed casting is produced with or without use of a binder, for instance, through the procedure shown in FIG. 1, it is difficult to obtain a fiber shaped body having a low fiber volume proportion (V.sub.f) of 10% or less, and even if one should succeed in shaping, deformation, cohesion, cracking, etc. would arise upon complexed casting (pressurized casting, vacuum casting, and the like), so that not only a desired strength cannot be realized but also a metallic member having stable quality cannot be produced. Also, it is difficult to obtain a fiber shaped body of V.sub.f =5% through the heretofore known process.
Now, with regard to structural members for use in an internal combustion engine, reduction of weight has been positively advanced by employing light alloy materials, and especially to form moving parts such as rocker arms to be used in an overhead camshaft type valve moving mechanism and pistons of light alloy materials is effective also for the purpose of reducing an inertial force.
However, in the case of employing light alloy materials, in order to supplement the shortage of the strength it becomes necessary to increase a wall thickness or to associate a reinforcing strong member therewith. Hence, according to that counter-measure for reinforcement, large-sizing and increase of weight of moving parts would be the result, and the counter-measure contradicts the essential object of realizing reduction in size.
Especially, a shaft fitting portion of a rocker arm which fits around a rocker arm shaft is required to have high fatigue strength, rigidity and abrasion-proofness, hence a counter-measure such as subjecting the inner wall of that portion to surface treatment or press-fitting a bush made of steel in the inner wall, must be taken, thus not only increasing of weight, but also steps of a manufacturing process are made more complex and a manufacturing cost becomes expensive.
Likewise, at a slide contact portion of a cam where high fatigue strength, rigidity and abrasion-proofness are required, it is necessary to integrally join an abrasion-proof member formed of iron series sintered material, ceramics or the like with a base metal layer through a method of cast wrapping or the like. However, in the case where an abrasion-proof member is formed of iron series sintered material that is heavy in weight, since the associated location is remote from the center of rocking motion, a moment of inertia is large hence it brings about a result that is contrary to the inherent objects (reduction in weight and reduction in an inertial force) of the formation of a rocker arm of light alloy materials. In addition, in the case where an abrasion-proof member is formed of ceramics, while reduction in weight can be contemplated as compared to iron series sintered material, there is a possibility that at the boundary portion between a base metal layer and the ceramics, a stress due to a difference in a coefficient of thermal expansion would arise, and hence unfavorable phenomena such as deformation, cracking, peal-off at the boundary surface, etc. may be generated.
On the other hand, a piston made of light alloys is light in weight, has a small inertia, and so, is suitable for high speed rotation. However, since the light alloy materials are generally poor in heat-resistivity, recently a technique for dealing with this problem by associating a heat-resisting member such as ceramics with a head portion of a piston for a high speed engine through the method of screw setting, cast wrapping or the like, has been employed. But, due to a large difference in thermal expansion between the heat-resisting member and the light alloy material forming the piston main body, there exists a disadvantage that thermal stress would arise at the boundary portion between these member and material, hence unfavorable phenomena such as cracking, deformation, etc. would be generated, and the durability of the piston is lowered.